Thermal protection mechanisms for uncooled microbolometers

ABSTRACT

Methods and apparatus for preventing solar damage, and other heat-related damage, to uncooled microbolometer pixels. In certain examples, a thermochroic membrane that becomes highly reflective at temperatures above a certain threshold is applied over at least some of the microbolometer pixels to prevent the pixels from being damaged by excessive heat.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of, and claims the benefits under 35U.S.C. §§ 120 and 121 of, co-pending U.S. patent application Ser. No.15/485,942 titled “THERMAL PROTECTION MECHANISMS FOR UNCOOLEDMICROBOLOMETERS” and filed on Apr. 12, 2017, which is hereinincorporated by reference in its entirety for all purposes.

BACKGROUND

Uncooled microbolometer detectors are used in a wide variety of infraredor thermal imaging applications. Uncooled microbolometer pixels arevulnerable to changes in resistance and other damage when they areexposed to heat sources. Excessive heat fundamentally changes theresponse in the pixel and degrades the resulting image. Extreme heatsources, such as the Sun, can completely destroy the pixels.Conventional practice is to avoid direct exposure of an uncooledmicrobolometer detector to the Sun. If solar damage does occur,re-calibration of the detector may be necessary to obtain a usableimage.

SUMMARY OF INVENTION

Aspects and embodiments are directed to methods and apparatus forpreventing solar damage, or other heat-related damage, to uncooledmicrobolometer pixels.

According to certain embodiments, at least some of the pixels of amicrobolometer are configured with a bimetallic thermal shortingstructure that uses thermal properties of dissimilar metals to trigger adeformation of part of the pixel structure to protect the pixel fromexcessive heat damage.

According to further embodiments, a thermochroic membrane is applieddirectly to the microbolometer pixel structure. This thermochroicmembrane becomes highly reflective at temperatures above a certainthreshold, and thus acts to prevent the pixels from being damaged byexcessive heat.

According to one embodiment an uncooled microbolometer includes a basesubstrate, a plurality of pixels arranged in an array on the basesubstrate, each pixel including a sensor layer supported above the basesubstrate by at least two first supports, and an infrared absorbinglayer supported above and thermally isolated from the sensor layer by atleast one second support, and at least one bimetallic switch coupled toa corresponding at least one pixel of the array of pixels, thebimetallic switch including a first layer of a first material and asecond layer of a second material, and being configured to thermallyshort the corresponding at least one pixel to the base substrate inresponse to a temperature of the corresponding at least one pixelreaching a predetermined threshold.

In one example the uncooled microbolometer further includes a groundcontact disposed on the base substrate, the at least one bimetallicswitch being configured to thermally short the corresponding at leastone pixel to ground via the ground contact. In another example, in aneutral state the at least one bimetallic switch is disposed parallelwith the infrared absorbing layer of the corresponding at least onepixel, and in a shorting state the at least one bimetallic switch isdeflected away from the infrared absorbing layer to thermally short thecorresponding at least one pixel to the base substrate. In one examplethe at least one bimetallic switch is coupled to the infrared absorbinglayer. In another example the at least one bimetallic switch is coupledto the second support. In one example the first material is titanium andthe second material is aluminum. In another example the first materialis titanium and the second material is silicon nitride. In anotherexample the first material is aluminum and the second material issilicon nitride. In one example the at least one bimetallic switchincludes a plurality of bimetallic switches, each bimetallic switch ofthe plurality of bimetallic switches being coupled to a correspondingone of the plurality of pixels. The uncooled microbolometer may furtherinclude a cap layer disposed over the plurality of pixels and coupled tothe base substrate, the cap layer being configured to provide a cavitybetween an first surface of the base substrate and a second surface ofthe cap layer, and the plurality of pixels being disposed within thecavity.

According to another embodiment an uncooled microbolometer includes abase substrate, a plurality of pixels arranged in an array on the basesubstrate, a cap layer coupled to and disposed over the base substrate,the cap layer being configured to provide a cavity between the basesubstrate and the cap layer, the plurality of pixels being disposedwithin the cavity, and a thermally sensitive protective membranedisposed on the cap layer over a sub-array of at least some of theplurality of pixels, the thermally sensitive protective membraneincluding a thermochroic switch material configured to transitionbetween a transmissive state and a reflective state in response to atemperature of thermochroic material reaching a predetermined threshold,the thermochroic material being transmissive to infrared radiation inthe transmissive state and reflective to the infrared radiation in thereflective state.

In one example the thermochroic material is a vanadium oxide material.In one example the vanadium oxide material is VO₂. The uncooledmicrobolometer may further include a cover layer disposed over thethermally sensitive protective membrane. In one example the cap layerand the cover layer are each made of silicon nitride. In one example thesub-array of at least some of the plurality of pixels includes a 5×5sub-array of pixels. In another example the sub-array of at least someof the plurality of pixels includes a 3×3 sub-array of pixels.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments are discussed in detail below. Embodimentsdisclosed herein may be combined with other embodiments in any mannerconsistent with at least one of the principles disclosed herein, andreferences to “an embodiment,” “some embodiments,” “an alternateembodiment,” “various embodiments,” “one embodiment” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide illustration and afurther understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of the invention. In the figures,each identical or nearly identical component that is illustrated invarious figures is represented by a like numeral. For purposes ofclarity, not every component may be labeled in every figure. In thefigures:

FIG. 1 is a cross-sectional schematic view of one example of amicrobolometer according to aspects of the present invention;

FIG. 2 is a cross-sectional view of a portion of a microbolometerincorporating a bimetallic switch, shown in the deflected position,according to aspects of the present invention;

FIG. 3A is a plan view of one example of the bimetallic switch of FIG.2, according to aspects of the present invention;

FIG. 3B is a cross-sectional view of the bimetallic switch of FIG. 3Ataken along the dashed line shown in FIG. 3A;

FIG. 4 is a graph showing deflection range (in micrometers) as afunction of temperature for examples of a bimetallic switchcorresponding to the structure and dimensions provided in FIGS. 3A and3B and Table 1, according to aspects of the present invention;

FIG. 5 is a cross-sectional view of a portion of a microbolometerincorporating a thermally sensitive protective membrane according toaspects of the present invention;

FIG. 6 is an enlarged view of a portion of the microbolometer of FIG. 5;and

FIG. 7 is a graph showing rise in temperature over time of pixels in anexample the microbolometer with different arrangements and materials ofthe protective membrane of FIGS. 5 and 6.

DETAILED DESCRIPTION

A microbolometer is a type of uncooled thermal sensor. Themicrobolometer includes an array of pixels, each pixel being made up ofseveral layers in what can be referred to as a “bridge” structure. Forexample, FIG. 1 is a cross-sectional view illustrating a portion of amicrobolometer 100 including a plurality of pixels 110 according tocertain embodiments. The microbolometer 100 includes a base substrate120 on which the array of pixels 110 are formed. The base substrate 120may be a silicon substrate, for example. Although not shown specificallyin FIG. 1, the base substrate 120 may include a read-out integratedcircuit (ROIC) for each pixel 110 or group of pixels. The ROIC processessignals received from each pixel 110 and provides output data that canbe used to construct an image, as discussed further below. A cap layer130, also referred to as a cap wafer, surrounds and encapsulates thearray of pixels 110, forming a cavity between the cap layer 130 and thebase substrate 120 in which the pixels 110 are disposed. The cavity istypically maintained under vacuum conditions to increase the longevityof the microbolometer 100. In certain examples the cap layer 130 is madeof silicon nitride (SiN).

Each pixel 110 includes a layer of infrared (IR) absorbing material 112and a sensor layer 114 that includes a thermal sensor or sensingcircuitry. The IR absorbing layer 112 is suspended above and thermallyisolated from the sensor layer 114 by supports 116. The sensor layer 114is itself supported above the base substrate 120 by supports 118. Thus,as shown in FIG. 1, the pixels 110 have a so-called “bridge” structure.In certain examples, the IR absorbing layer 112 can be suspendedapproximately 2 microns (μm) above the sensor layer substrate 114.Because the microbolometer 100 does not undergo any cooling, the IRabsorbing material 112 must be thermally isolated from the ROIC on thebase substrate 120, and the bridge-like structure allows for this tooccur.

In certain examples the supports 118 can provide electrical contacts forthe pixels. Thus, in the illustrated example, each pixel 110 includes apair of supports 118, including a first support 118 a that can beconnected to a reference potential, such as ground, and a second support118 b that can be connected to an electrical contact 122 on the basesubstrate 120 such that the sensing circuitry on the sensor layer 114can be connected to the ROIC on the base substrate 120. In the exampleshown in FIG. 1, the electrical contact 122 includes a via extendingthrough the base substrate 120. This can allow the ROIC associated witheach pixel 110 (or group of pixels) to be connected to externalcircuitry (not shown), such that image data or other information can betransferred from the microbolometer 100 to external devices, such asimage processors, memory, or other electronic devices.

Uncooled microbolometers produce images from thermal radiation through aknown relationship between the resistance of each pixel 110 in themicrobolometer 100 and the intensity of the received thermal radiation.In order to achieve satisfactory imaging resolution, the material usedin the IR absorbing layer 112 should demonstrate large changes inresistance as a result of minute changes in temperature. Therelationship between the change in resistance and the heat absorbed bythe IR absorbing layer 112 is defined at least in part by thetemperature coefficient of resistance (TCR) of the material. In certainexamples, as the IR absorbing layer 112 is heated, due to incominginfrared radiation, the resistance of the material decreases. This isthe case for materials with a negative temperature coefficient. Twomaterials that are commonly used as infrared detecting materials inmicrobolometers are amorphous silicon and vanadium oxide.

Recent studies have shown that when a microbolometer pixel is exposed todirect sunlight, the heat from solar energy causes the resistance of themicrobolometer pixel to change in a way that causes permanentdegradation in the performance (e.g., imaging resolution) of the pixel.Solar damage to microbolometers can be a particular problem fordistributed aperture imaging systems because the Sun can often be in thefield of view of such a system. Accordingly, it would be highlyadvantageous to provide microbolometers with mechanisms by which solardamage can be avoided or mitigated.

According to certain embodiments, a thermally sensitive bimetallicswitch is integrated into the microbolometer pixel structure andconfigured to short, e.g. thermally short, the respective pixel toground, for example, if the temperature of the pixel reaches a certainthreshold. When any one or more pixels 110 of the microbolometer 100 areexposed to a potentially damaging heat source, such as the Sun or apowerful laser beam, for example, the IR absorbing layer 112 absorbs theheat, and the temperature of the pixel rises rapidly. According tocertain embodiments, any or all pixels 110 of the microbolometer 100 canbe provided with a thermally sensitive bimetallic switch configured toprevent the temperature of the pixel from rising above a certainthreshold. The threshold can be selected such that the pixel is notpermanently damaged at the threshold temperature. The materials of thebimetallic switch can be selected such that the switch is sensitive totemperature, and is activated at the threshold temperature to short thepixel to ground or to the base substrate, thereby providing a thermal“sink” and preventing the temperature of the pixel from risingsignificantly further and damaging the pixel.

Referring to FIG. 2 there is illustrated a pixel 110 a incorporating anexample of such a thermally sensitive bimetallic switch 200. Thebimetallic switch 200 is attached to the IR absorbing layer 112 and/orthe thermally isolating support 116. In the neutral state (i.e. atnormal operating temperatures and below), the bimetallic switch 200 maylie parallel with the IR absorbing layer 112. The materials ofbimetallic switch 200 can be selected such that the bimetallic switch200 deflects or bends away from the IR absorbing layer 112 as thetemperature of the pixel 110 a rises. In certain examples the bimetallicswitch 200 is configured such that when the temperature of the pixel 110a reaches a certain threshold, the bimetallic switch 200 is deflectedsufficiently to contact the support 118 a connected to ground, as shownin FIG. 2, thereby shorting the pixel 110 a to ground. In otherexamples, a ground plane or ground contact can be provided on the basesubstrate 120, and the bimetallic switch 200 can be configured tocontact that ground plane or contact (not necessarily the support 118 a)when the temperature of the pixel 110 a reaches the threshold. In otherexamples the bimetallic switch 200 can be configured to contact the basesubstrate 120 directly, thereby shorting the pixel 110 a to the basesubstrate 120 and not necessarily to ground.

The configuration of the bimetallic switch 200 may include itsdimensions and materials. Referring to FIGS. 3A and 3B there isillustrated one example of the bimetallic switch 200. The bimetallicswitch 200 includes a layer of a first material 210 and a layer of asecond material 220. In one example the first material 210 is titanium(Ti) and the second material 220 is aluminum (Al). In another examplethe first material 210 is aluminum and the second material 220 issilicon nitride. In another example the first material 210 is titaniumand the second material is silicon nitride. It certain embodiments, itcan be advantageous to select titanium, aluminum, or silicon nitride asmaterials for the bimetallic switch 200 because these materials arecommonly used in the manufacturing processes of the microbolometer 100.In addition, these materials allow the bimetallic switch 200 to deflectand short the pixel 110 a to ground at threshold temperatures that areabove the typical operating range for the microbolometer 100 (such thatthe presence of the bimetallic switch does not limit the imaging rangeor performance), but well below the levels at which significant orpermanent damage to the pixel 110 a occurs. However, those skilled inthe art will appreciate, given the benefit of this disclosure, thatembodiments of the bimetallic switch are not limited to using titanium,aluminum, or silicon nitride, and other materials may be selected.

The dimensions (length, width, thickness) of the bimetallic switch 200may be selected based on the desired range of deflection to be achievedand the size of the pixel 110 a. For example, the bimetallic switch 200should have a thickness T and a width W both sufficiently thin such thatan increase in temperature is able to cause deflection of the bimetallicswitch 200 to achieve the shorting function discussed above. Inaddition, the thickness and the width should be sufficient such that thebimetallic switch 200 provides a good thermal shorting path. In certainexamples the thickness of the bimetallic switch 200 is in a range of0.004 μm to 0.175 μm. In certain examples the width of the bimetallicswitch 200 is 3 μm. Further, the length L of the bimetallic switch 200should be sufficient such that, in the fully deflected state as shown inFIG. 2, the bimetallic switch 200 contacts the grounding support 118 aor grounding plane/contact on the base substrate 120. Thus, the lengthof the bimetallic switch 200 may depend at least in part on the size ofthe pixel 110 a, in particular, the distance between the IR absorbinglayer 112 and the base substrate 120 and the “pitch” or spacing betweenadjacent pixels. In certain examples in which the pixel pitch is 12 μm,the bimetallic switch has a length of 11 μm.

FIG. 4 is a graph showing deflection (in μm) as a function oftemperature for various examples of the bimetallic switch 200. In eachexample the bimetallic switch has a length of 11 μm and a width of 3 μm.For each example, the first and second materials 210, 220, are each oneof titanium, aluminum, and silicon nitride, and the thickness of eachmaterial is provided in Table 1 below. Line 300 indicates a desiredlevel of deflection. In the example of FIG. 4, the desired level ofdeflection is shown at 3 μm, corresponding to an example of a pixel 110a in which the IR absorbing layer 112 is positioned approximately 3 μmabove the base substrate 120. However, in other examples the desiredlevel of deflection may be different depending on, for example,different dimensions of the pixel 110 a or different positioning of thebimetallic switch 200.

TABLE 1 Ti Thickness Al Thickness SiN Thickness Example (μm) (μm) (μm) 10.035 0.07 N/A 2 0.035 0.14 N/A 3 0.018 0.04 N/A 4 0.035 0.035 N/A 50.02 0.035 N/A 6 0.02 0.02 N/A 7 N/A 0.07 0.035 8 0.02 0.035 N/A 9 N/A0.035 0.02 10 0.02 N/A 0.02 11 0.035 N/A 0.02

The examples shown in FIG. 4 demonstrate that various configurations ofthe bimetallic switch 200 can deflect sufficiently within an appropriatetemperature range to achieve the desired thermal shorting function.

Further aspects and embodiments are directed to the use of a thermallysensitive protective membrane that can be applied on the microbolometerpixel structure to prevent the microbolometer from being damaged whendirect solar energy is in the field of view.

Referring to FIG. 5 there is illustrated a cross-sectional view of aportion of a microbolometer 100 a including a protective membrane 400integrated into the microbolometer structure. FIG. 6 shows an enlargedview of a portion of the microbolometer 100 a of FIG. 5. As shown inFIG. 6, in one example the microbolometer 100 a has a height H ofapproximately 6.975 μm, and the protective membrane 400 is applied overthe cap layer 130 at a distance D above the IR absorbing layer 112. Inthis example, D is approximately 1.6 μm. The protective membrane 400 mayinclude a thermochroic switch material, such as vanadium oxide, to actas a barrier to heat from the Sun or another intense heat source, suchas a laser, for example, and prevent damage from exposure to such a heatsource. Thermochroic switch materials, such as vanadium oxide, havematerial properties that cause the material to become highly reflectivewhen elevated to temperatures above a certain threshold. These samematerials are highly transmissive at temperatures below the threshold.For example, the VO₂ undergoes a metal-insulator phase change nearapproximately 67 degrees Celsius (° C.), and thus is highly transmissiveto infrared radiation below this temperature threshold and highlyreflective above it. Accordingly, by applying a coating or film of sucha thermochroic material over the pixels 110 of the microbolometer 100 a,as shown in FIGS. 5 and 6, excessive solar heat that would otherwisechange the resistance value of the microbolometer pixels 110 andpotentially damage the pixels 110 can be reflected away by theprotective membrane 400 before any damage to the pixels 110 occurs,without compromising the imaging performance of the array. Other phasesof vanadium oxide, and other thermochroic switch materials may undergo aphase change at different temperature thresholds. Thus, in certainexamples the material of the protective membrane 400 can be chosen basedon a suitable temperature threshold at which the material changes frombeing transmissive in a certain spectral range to reflective in thatspectral range. In certain examples, a vanadium oxide material, such asVO₂, may be a desirable choice because other vanadium oxide materialsare commonly used for other components in the microbolometer 100 a andcan be easily integrated into a CMOS fabrication process used tomanufacture the microbolometer 100 a.

According to certain embodiments, in the microbolometer 100 a, thepixels 110 are arranged in rows and columns as a two-dimensional array.FIG. 5 illustrates an example of a row (or column) of five pixels 110.According to certain examples, the protective membrane 400 may beapplied over all the pixels 110 of the microbolometer 100 a as acontinuous film. In other examples the protective membrane 400 can beapplied as one or more discrete film “patches” over group(s) of pixels110. For example, the protective membrane 400 can include one or morefilms provided over corresponding one or more sub-arrays of 3×3 pixels110 or 5×5 pixels.

Still referring to FIGS. 5 and 6, in certain examples, a cover layer 140can be applied over the protective membrane 400 to add structuralsupport and protection for membrane 400. The cover layer 140 can be madefrom the same material as the cap layer 130. For example, the coverlayer 140 may be made of silicon nitride.

FIG. 7 is a graph showing the rise in temperature over time of thepixels 110 of an example of the microbolometer 100 a for differentarrangements and materials of the protective membrane 400. Tables 2 and3 below provide parameters corresponding to the examples shown in FIG.7. Table 2 applies to all four examples shown in FIG. 7 and described inTable 3.

TABLE 2 Conductivity Density Specific Heat Layer Material (W/m-K)(g/cm³) (J/g-K) 130 SiN 1.24 1.38 0.9 400 VO₂ varied 1.38 0.9

TABLE 3 Membrane Steady-State Example Pixel conductivity Temperature #Arrangement (W/m-K) (° C.) 1 5 × 5 10 105.5 2 5 × 5 1 177.1 3 3 × 3 1054.5 4 3 × 3 1 76.5

Referring to FIG. 7, as infrared radiation is received and absorbed bythe pixels 110 of the microbolometer 100 a, the temperature of thepixels 110 rises. However, due to the presence of the protectivemembrane 400, at a certain threshold, the pixels 110 reach a “steadystate” temperature, above which the temperature of the pixels 110 doesnot appreciably rise further. As may be seen with reference to Table 3and FIG. 7, the steady state temperature depends on various factors,including the arrangement of the pixels 110 over which the protectivemembrane 400 is applied and the thermal conductivity of the protectivemembrane 400.

In Example 1 (corresponding to curve 510), a protective membrane 400 ofVO₂ was applied over a 5×5 array of pixels 110, and the protectivemembrane 400 was configured with a conductivity of 10 Watts permeter-Kelvin (W/m-k). As infrared radiation is received by the pixels110, the temperature of the pixels 110 rises over time, until reaching athreshold temperature at which the thermochroic switch material of theprotective membrane 400 changes from being substantially transmissive tosubstantially reflective, and the protective membrane 400 starts toreflect the infrared radiation away from the pixels 110, preventingfurther significant increase in the temperature of the pixels 110. InExample 1, the pixels 110 reached a steady state temperature ofapproximately 105.5° C. after approximately 5 microseconds (μs).

In Example 2 (corresponding to curve 520), the protective membrane 400of VO₂ was similarly applied over a 5×5 array of pixels 110, but wasconfigured with a thermal conductivity of only 1 W/m-K. As shown in FIG.7, in this example, the pixels 110 reached a steady state temperature ofapproximately 177° C. after approximately 9 μs.

In Examples 3 and 4, the protective membrane 400 of VO₂ was applied overa smaller 3×3 array of pixels 110. In Example 3 (corresponding to curve530) the protective membrane 400 was configured with a thermalconductivity of 10 W/m-K (as in Example 1), and in Example 4(corresponding to curve 540) with a thermal conductivity of 1 W/m-K (asin Example 2). As shown in FIG. 7, in Example 3 the pixels 110 reached asteady state temperature of approximately 54.5 degrees afterapproximately 2 μs, and in Example 4 the pixels 110 reached a steadystate temperature of approximately 76.5 degrees after approximately 3μs.

These examples show that a lower steady state temperature can beachieved by applying the protective membrane 400 over smaller sub-arraysof pixels 110 in the microbolometer 100 a. Thus, the size of the pixelsub-arrays over which the protective membrane 400 is applied may beselected based at least in part on a desired range or approximate targetvalue of the steady state temperature. It may be desirable that thesteady state temperature is sufficiently high so as not to limit thethermal imaging range or performance of the microbolometer 100 a, butsufficiently low such that the pixels 110 are not damaged by incidentlight. Further, the thermal conductivity of the thermochroic switchmaterial used for the protective membrane 400 determines the uniformityof the temperature distribution of the protective membrane 400. A higherthermal conductivity of the protective membrane 400 results in a lowersteady state temperature, other parameters being equivalent.

Thus, aspects and embodiments provide mechanisms by which solar damage,or damage from other intense heat/light sources, can be prevented inuncooled microbolometers. These mechanisms can be integrated into thepixel structure of the microbolometer to provide automatic protectionwithout requiring additional devices external to the microbolometers. Asdiscussed above, in certain examples a protective membrane made of athermochroic switch material can be applied to the pixel bridgestructure during manufacture of the microbolometer. The presence of theprotective membrane allows the microbolometer to be inherently immune todamage from solar exposure through the action of the thermochroic switchmaterial. In other examples, a bimetallic switch can be integrated intothe pixel bridge structure to thermally short the pixels to thesubstrate, or other structure that can act as a thermal sink, in thepresence of excessive heat. Thus, an increase in the scene or ambienttemperature can be used to activate a mechanism to either deflectexcessive energy away using the thermochroic membrane or thermally shortthe pixels using the bimetallic switch to mitigate excessive heat thatcould otherwise damage the pixels. The incorporation of such protectivemechanisms may allow uncooled microbolometers to be used forapplications where they have not been traditionally used due tovulnerability to solar damage, such as DAS systems for ground andairborne applications.

Having described above several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure and are intended to be within the scope of the invention.Accordingly, it is to be appreciated that embodiments of the methods andapparatuses discussed herein are not limited in application to thedetails of construction and the arrangement of components set forth inthe foregoing description or illustrated in the accompanying drawings.The techniques and mechanism disclosed herein are capable ofimplementation in other embodiments and of being practiced or of beingcarried out in various ways. Examples of specific implementations areprovided herein for illustrative purposes only and are not intended tobe limiting. Also, the phraseology and terminology used herein is forthe purpose of description and should not be regarded as limiting. Theuse herein of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof is meant to encompass the itemslisted thereafter and equivalents thereof as well as additional items.References to “or” may be construed as inclusive so that any termsdescribed using “or” may indicate any of a single, more than one, andall of the described terms. Any references to front and back, left andright, top and bottom, upper and lower, and vertical and horizontal areintended for convenience of description, not to limit the presentsystems and methods or their components to any one positional or spatialorientation. Accordingly, the foregoing description and drawings are byway of example only, and the scope of the invention should be determinedfrom proper construction of the appended claims, and their equivalents.

What is claimed is:
 1. An uncooled microbolometer comprising: a basesubstrate; a plurality of pixels arranged in an array on the basesubstrate; a cap layer coupled to and disposed over the base substrate,the cap layer being configured to provide a cavity between the basesubstrate and the cap layer, the plurality of pixels being disposedwithin the cavity; and a thermally sensitive protective membranedisposed on the cap layer over a sub-array of at least some of theplurality of pixels, the thermally sensitive protective membraneincluding a thermochroic switch material configured to transition from atransmissive state into a reflective state in response to a temperatureof thermochroic material reaching a predetermined threshold, thethermochroic material being transmissive to infrared radiation in thetransmissive state and reflective to the infrared radiation in thereflective state.
 2. The uncooled microbolometer of claim 1 wherein thethermochroic switch material is vanadium oxide.
 3. The uncooledmicrobolometer of claim 2 wherein a phase of the vanadium oxide is VO₂that undergoes a metal-insulator phase change at a temperature ofapproximately 67 degrees Celsius.
 4. The uncooled microbolometer ofclaim 1 further comprising a cover layer disposed over the thermallysensitive protective membrane.
 5. The uncooled microbolometer of claim 4wherein the cap layer and the cover layer are made of silicon nitride.6. The uncooled microbolometer of claim 1 wherein the thermallysensitive protective membrane is a continuous film disposed over all theplurality of pixels.
 7. The uncooled microbolometer of claim 1 whereinthe sub-array of at least some of the plurality of pixels includes a 5×5sub-array of pixels.
 8. The uncooled microbolometer of claim 1 whereinthe sub-array of at least some of the plurality of pixels includes a 3×3sub-array of pixels.
 9. The uncooled microbolometer array of claim 1wherein each pixel includes a sensor layer supported above the basesubstrate by at least two first supports, and an infrared absorbinglayer supported above and thermally isolated from the sensor layer by atleast one second support.
 10. The uncooled microbolometer array of claim9 further comprising: a read-out integrated circuit formed in the basesubstrate and coupled to at least one of the at least two first supportsof each pixel, the read-out integrated circuit being configured toreceive and process signals from the plurality of pixels to provideoutput data for constructing an image.
 11. The uncooled microbolometerarray of claim 9 wherein the infrared absorbing layer is one ofamorphous silicon and vanadium oxide.
 12. The uncooled microbolometerarray of claim 1 wherein the cavity is maintained under vacuumconditions.
 13. An uncooled microbolometer comprising: a base substrate;a plurality of pixels arranged in a plurality of two-dimensionalsub-arrays on the base substrate; a cap layer coupled to and disposedover the base substrate, the cap layer being configured to provide acavity between the base substrate and the cap layer, the plurality ofpixels being disposed within the cavity; and a thermally sensitiveprotective membrane disposed on the cap layer, the thermally sensitiveprotective membrane including a plurality of discrete portions, eachportion being disposed over a corresponding one of the plurality ofsub-arrays of the plurality of pixels, the thermally sensitiveprotective membrane including a thermochroic switch material configuredto transition from a transmissive state into a reflective state inresponse to a temperature of thermochroic material reaching apredetermined threshold, the thermochroic material being transmissive toinfrared radiation in the transmissive state and reflective to theinfrared radiation in the reflective state.
 14. The uncooledmicrobolometer of claim 13 wherein the thermochroic switch material isvanadium oxide.
 15. The uncooled microbolometer of claim 13 furthercomprising a cover layer disposed over the thermally sensitiveprotective membrane.
 16. The uncooled microbolometer of claim 15 whereinthe cap layer and the cover layer are made of silicon nitride.
 17. Theuncooled microbolometer of claim 13 wherein each sub-array is a 5×5sub-array of pixels.
 18. The uncooled microbolometer of claim 13 eachsub-array is a 3×3 sub-array of pixels.
 19. The uncooled microbolometerarray of claim 13 wherein each pixel includes a sensor layer supportedabove the base substrate by at least two first supports, and an infraredabsorbing layer supported above and thermally isolated from the sensorlayer by at least one second support.
 20. The uncooled microbolometerarray of claim 19 wherein the infrared absorbing layer is one ofamorphous silicon and vanadium oxide.